The invention relates to a circuit for linearly amplifying and for demodulating an AM-modulated signal. Such circuits are known and are frequently used, for example, in broadcasting receivers for amplifying and demodulating an intermediate-frequency AM-modulated audio or video signal.
It is important that the gain remains linear over a large dynamic range, that is to say, the amplifier should be prevented from performing its limiting action in the case of a large signal field strength. For this purpose it is conventional practice to use automatic gain control. In the case of larger signal field strengths the gain of a stage is reduced so that the subsequent stages are prevented from performing their limiting actions.
However, automatic gain control has its drawbacks. A plurality of circuit elements is required for deriving, postponing, amplifying and/or smoothing the required control voltage. Amplifier stages must be controllable for which concessions must be made to the noise and/or distortion properties of the amplifier stages. Moreover, an effect of the AVC is that the lowest frequencies of the modulation signals are partly suppressed, resulting in an undesirable attenuation of the demodulated signal at the low end of the frequency characteristic.